This invention relates generally to inverter synchronous machine drive systems, and more specifically, to an improved control apparatus for operating an inverter synchronous machine drive system at maximum machine efficiency with reduced machine reactive power.
Inverter-synchronous machine drive systems are commonly employed in industrial applications, such as glass making, textile processing and the like, where a wide range of machine speed control is desired. Typically, such inverter-synchronous machine drive systems are comprised of a multi-phase synchronous machine which is excited through an inverter from a direct current source, usually a phase controller rectifier. Preferably, the inverter comprises a plurality of pairs of controlled switching devices, such as thyristors, the number of pairs of thyristors corresponding to the number of machine phases, with each of the thyristors of each pair coupled in series-aiding fashion, and each of the serially coupled pairs of thyristors coupled across the direct current source. When the thyristors of each pair are rendered conductive in a prescribed sequence, alternating current is supplied to the machine by the inverter. By regulating the inverter thyristor firing frequency, the inverter output frequency and hence machine speed can be controlled accordingly. Varying the amplitude of the link current supplied by the phase controlled rectifier varies the inverter output current amplitude.
Heretofore, control devices employed to regulate inverter thyristor conduction have been comprised of dedicated hardware designed to maintain a fixed turn-off angle, .delta., the turn-off angle being the angular period during which a reverse bias voltage is applied across the then-conductive or outgoing inverter thyristor to render it nonconductive. Fixing the inverter turn-off angle incurs no difficulty if inverter frequency is fixed, as is the case for fixed speed drives. This is because the inverter advance angle, .delta. that is, the phase delay between initiation of incoming inverter thyristor conduction and the zero crossing of machine phase voltages, can be set to minimize machine reactive power. However, when inverter frequency is varied, as is the case in variable-speed, inverter-synchronous machine drives, fixing the inverter turn-off angle to achieve minimum reactive currents at maximum machine frequencies incurs the problem that at lower speeds the fixed inverter turn-off time corresponding to the fixed turn-off angle is larger than necessary, causing the machine to operate at an unnecessarily low power factor and thereby increasing machine reactive power. Also, prior art inverter-synchronous machine control devices commonly operate to achieve maximum synchronous machine efficiency by regulating inverter link current, that is, the current drawn by the inverter from the direct current source, in accordance with a fixed relationship between inverter link current and air gap flux. The principle of operation of such prior art control devices is based on the assumption that machine load torque varies in a predetermined relationship with machine frequency, machine parameters remain constant and that the form factor of the machine voltage, machine current and machine flux waveforms remains constant. Unfortunately, such ideal conditions hardly ever exist during operation of a typical inverter-synchronous machine drive system under normal circumstances and hence, maximum machine efficiency is usually not achieved by the use of such prior art control devices.
In contrast to such prior art control devices, the inverter-synchronous machine drive system control apparatus of the present invention optimizes inverter thyristor turn-off time irrespective of machine frequency, thereby minimizing inverter-machine drive system reactive power. Additionally, the control apparatus of the present invention regulates inverter link current and machine field current in accordance with real time machine efficiency to assure inverter-synchronous machine drive system operation at maximum efficiency irrespective of machine speed and torque.